DMVPN

NBMA: nonbroadcast multiaccess

Turning off split horizon is needed on Tunnel interface if any routing protocol is used:

When the device is connected to a spoke and not only the HUB (and some routing protocol is running) then split horizon causing problems.
When the device receives an update for example from the HUB it cannot forward the routing update towards another spoke because it should have send it on the same interface (on the tunnel interface).